I. Field of the Invention
This invention relates generally to apparatus for detecting flaws in tires and more specifically to an improved design of an ultrasonic tire testing system for detecting the presence of holes and/or separations in tires for wheeled vehicles.
II. Discussion of the Prior Art
In the tire re-tread industry non-destructive, visual testing is performed on used tire carcasses to ensure that they are free from holes and separations. It is essential that the testing be accomplished in a minimum of time, yet with reliability standards maintained, whereby defective carcasses can be culled from those being selected for retreading.
Visual inspection is reasonably fast, however, separations and holes are missed due either to human error or to the lack of visible evidence of separations in high tread, radial or other casings.
For a number of years, now, machines have been developed to perform non-destructive tests on tires through the use of ultrasonic inspection techniques. For example, in the McCauley U.S. Pat. No. 3,882,717 there is disclosed an arrangement in which the tire to be inspected is placed in a liquid "coupling" medium and ultrasonic energy is transmitted thru the material under test and that energy passing through the tire is picked up by suitably oriented receiving transducers. By observing variations in the amplitude of the received signals, it can be determined whether voids have been scanned as the tire is rotated past the transmitting/receiving transducers. This is the so-called "through transmission" method.
The apparatus and method of the McCauley Patent suffer from a number of drawbacks. First of all, the need for a liquid coupling medium makes the technique undesirable in that the subsequent handling of the wet tire tends to create a mess and is distasteful to the human operator. Furthermore, a subsequent drying step is required before further retreading steps can be accomplished. In addition, several revolutions of the tire are required in order to scan its entire surface. It is, of course, desirable to be able to cover the entire surface in a single revolution of the tire carcass under test, irrespective of its width and diameter, in order to reduce test time.
In the Halsey U.S. Pat. No. 4,059,989 there is described a method and apparatus for performing non-destructive flaw tests on vehicle tires wherein the need for a liquid ultrasound coupling medium is obviated. In this arrangement, thru-transmission is also used but the transmitting transducer and the receiving elements are placed in contact with the surface of the tire.
The rubbing contact between the transducers and the tire carcass creates a number of problems. First of all, the surface of the tire tends to be dirt laden and the scraping of the transducer across the tire surface usually causes dirt and dust to foul the transducer element resulting in the need for frequent and costly maintenance. Furthermore, rubbing surfaces create sonic energy and vibration which, when received by the receiving transducers may distort or otherwise affect the reliability of the test apparatus. The frictional engagement also leads to premature wear on the transducer. Foreign bodies, such as small stones or nails sticking through the tire may ruin the transducers.
The present invention obviates all of the above-mentioned drawbacks of prior art ultrasonic tire testing equipment. Specifically, the apparatus of the present invention does not require a liquid coupling medium, nor must the transmitting and/or receiving transducers be in direct contact with the tire carcass. Furthermore, the entire surface of most vehicle tires can be scanned in a single revolution of the tire, thus speeding up the overall rate at which a tire carcass may be fully inspected.
While dispensing with the need for a liquid coupling medium in the ultrasonic test unit is highly advantageous, the use of non-contact air coupling between the transmitting transducers, the object under test and the receiving transducers raise a number of ancillary problems. Specifically, the receiving electronics must be capable of distinguishing between the signal transmitted through the object under test and so-called "talk-around" ultrasonic waves which reach the receiving transducers by an indirect path. Also, the positioning of the transmitting array with respect to the object under test may be critical in that standing waves may be created which tend to render the overall system unduly sensitive to variations in geometry. The present invention uses pulsed transmitters and gated receivers with timing specially related to transmitter/receiver/casing geometry.
To permit casing inspection in one rotation multiple transmitters and receivers are utilized to simultaneously illuminate and inspect the casing in the width dimension while the casing rotates about its axis. A problem attendant in such systems which use multiple transmitters is the interference between the individual transducers comprising the transmitting array. The present invention uses frequency separation and transmitter array geometries which reduce this interference.
Most ultrasonic tire inspection systems have problems created by the tread pattern on the casing causing received signals which interfere with the flaw detection. This invention uses a finite sampled data filter to deal with this problem.